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Newborn Neurons in the Olfactory Bulb Selected for
Long-Term Survival through Olfactory Learning Are
Prematurely Suppressed When the Olfactory Memory Is
Erased
Sébastien Sultan, Nolwen Rey, Joëlle Sacquet, Nathalie Mandairon, Anne
Didier
To cite this version:
Brief Communications
Newborn Neurons in the Olfactory Bulb Selected for
Long-Term Survival through Olfactory Learning Are
Prematurely Suppressed When the Olfactory Memory Is Erased
Se´bastien Sultan,* Nolwen Rey,* Joelle Sacquet, Nathalie Mandairon, and Anne Didier
Institut National de la Sante´ et de la Recherche Me´dicale (INSERM) 1028, CNRS 5292, Lyon Neuroscience Research Center, Neuroplasticity and Neuropathology of Olfactory Perception Team, Universite´ Lyon1, Villeurbanne, F-Universite´ de Lyon, F-69007 Lyon, France
A role for newborn neurons in olfactory memory has been proposed based on learning-dependent modulation of olfactory bulb
neuro-genesis in adults. We hypothesized that if newborn neurons support memory, then they should be suppressed by memory erasure. Using
an ecological approach in mice, we showed that behaviorally breaking a previously learned odor–reward association prematurely
sup-pressed newborn neurons selected to survive during initial learning. Furthermore, intrabulbar infusions of the caspase pan-inhibitor
ZVAD (benzyloxycarbonyl-Val-Ala-Asp) during the behavioral odor–reward extinction prevented newborn neurons death and erasure
of the odor–reward association. Newborn neurons thus contribute to the bulbar network plasticity underlying long-term memory.
Introduction
The mammalian olfactory bulb (OB) is with the dentate gyrus of
the hippocampus, one of the two adult brain structures
undergo-ing permanent neurogenesis (Lledo et al., 2006). It is clear from
recent literature that newborn neuron survival in the OB is
mod-ulated by learning (Alonso et al., 2006; Mandairon et al., 2006b;
Mouret et al., 2008; Moreno et al., 2009; Sultan et al., 2010, 2011).
However, the role of adult-born neurons in memorization
and/or retention of olfactory information is still debated.
Asso-ciative olfactory learning is known to increase the survival of
adult-born neurons in the granule cell layer of the OB during
their critical period of synaptic integration to the pre-existing
network (Alonso et al., 2006; Mandairon et al., 2006b; Mouret et
al., 2008; Sultan et al., 2010), suggesting they play a role in
mem-ory. Moreover, the long-term survival of these newborn neurons
has been closely correlated with the duration of memory (Sultan
et al., 2010). To analyze the necessity of newborn neurons in the
process of learning and recall, several recent studies have blocked
neurogenesis using various methods and looked at the functional
outputs of this blockade. Two studies designed to block
prolifer-ation by infusion of the antimitotic drug AraC or irradiprolifer-ation
(both targeting the subventricular zone), reported a deficit in the
retention of an associative learning task. These findings suggested
a role for newborn neurons in supporting long-term memory
(Lazarini et al., 2009; Sultan et al., 2010). This conclusion was
reinforced by the preferential functional activation of newborn
neurons upon recall of the task (Sultan et al., 2010). However,
two other studies using genetic ablation of neurogenesis or AraC
treatment reported no alteration in learning or long-term
reten-tion of an associative olfactory task (Imayoshi et al., 2008;
Breton-Provencher et al., 2009). These discrepancies could arise from
differences in the learning paradigms used (Mandairon et al.,
2011). To address this issue of the role of adult-born neurons in
olfactory memory, we chose a new approach consisting of
mod-ulating the memory trace rather than the rate of neurogenesis. If
newborn neurons support long-term memory, then erasing the
memory should impair long-term survival of these newborn
neu-rons. One ecological way of challenging this hypothesis is to
cre-ate a specific memory trace associating an odor and a reward, to
then behaviorally break this association, and then to check
whether the newborn neurons were suppressed when the learned
significance of the odor was lost.
Materials and Methods
Animals
Eighty male C57BLack6/J mice (Charles River) aged 8 weeks at the be-ginning of the experiments were used. All mice were housed under a 12 h light/dark cycle in an environmentally controlled room and had free access to water and food except during the olfactory learning period. All behavioral training was conducted in the afternoon (2:00 P.M. to 5:00 P.M.). The protocol was approved by the Lyon 1 ethics committee (pro-tocol number BH2010 –35), and every effort was made to minimize both the number of animals used and their suffering during the experimental procedure in accordance with the European Community Council Direc-tive of November 24, 1986 (86/609/EEC).
Behavioral experiments
Experimental setup
All mice were tested on a computer-assisted two-hole board apparatus run by specific software (Mandairon et al., 2006b, 2009). The trial started by placing the mouse on the board, and the sequence of nose-poking into
Received July 19, 2011; accepted Aug. 12, 2011.
Author contributions: S.S., N.M., and A.D. designed research; S.S., N.R., and J.S. performed research; S.S., N.R., N.M., and A.D. analyzed data; S.S., N.M., and A.D. wrote the paper.
*S.S. and N.R. contributed equally to this work.
This work was supported by CNRS and Claude Bernard Lyon1 University. The authors declare no competing financial interests.
Correspondence should be addressed to Anne Didier, Lyon Neuroscience Research Center, INSERM 1028, CNRS 5292, Universite´ de Lyon, 50, Avenue Tony Garnier, F-69007 Lyon, France. E-mail: didier@olfac.univ-lyon1.fr.
DOI:10.1523/JNEUROSCI.3677-11.2011
the holes was automatically recorded. A polypro-pylene swab was placed at the bottom of the hole, covered with bedding, and impregnated with 20 l of pure odorant for trials involving odors.
Odorants
The odorants ⫹limonene (purity ⬎ 97%, Sigma-Aldrich) and ⫹carvone (purity 96%, Aldrich) were used.
Associative learning
Shaping. The mice were first trained to retrieve
a reward (a small piece of cereal; Kellogg) by digging through the bedding. The mouse was put in the start area and allowed to dig for 2 min. Shaping was considered to be complete when a mouse could successfully retrieve a re-ward buried deep in the bedding.
Olfactory conditioning. During the olfactory
learning experiments, mice were food deprived (to get a 10% reduction in body weight) begin-ning 5 d before the shaping session. Condition-ing consisted of five sessions (one per day) of four trials (2 min per trial; intertrial interval: 15 min). Each mouse was placed on the board and had to retrieve the reward that was now sys-tematically associated with the odorant. To avoid spatial learning, this was randomly placed in one of the two holes; the other con-tained no odorant and no reward.
Visual conditioning. The above procedure
was repeated except that the cue was no longer the odorant but a small red cap placed at the edge of the hole containing the reward.
In both visual and olfactory conditioning, randomization was controlled to avoid three trials in a row in which the reward would be placed in the same hole as well as strict alterna-tion of the place of the reward.
Pseudo-conditioning. In the olfactory and
vi-sual pseudo-conditioned groups, the olfactory or visual cues were randomly associated with the hole containing the reward. Randomization was controlled as in conditioned animals. In addition, the configuration in which the reward would or would not be associated by chance to the cue over three trials of a session was suppressed.
Olfactory retention test. Nine days after the last
olfactory conditioning session (and consequently 4 d post-visual conditioning) a retention test was performed, using the same conditions as during olfactory conditioning to check olfactory memo-rization of the original task.
Data analysis. For each trial, latency (time to
find the reward) was recorded. For each behioral session, mean latency was calculated and av-eraged within groups. All results are given as mean ⫾ SEM. Learning was assessed using ANOVA for repeated measures (Systat software, SSI), and pair comparisons were made using unilateral t tests. Statistical significance was set at p⬍ 0.05.
Intrabulbar infusions of benzyloxycarbonyl-Val-Ala-Asp
Mice (n⫽ 20) were anesthetized with cocktail injection of 50 mg/kg ket-amine and 7.5 mg/kg xylazine (intraperitoneally), secured in a stereotaxic instrument (NSI), and implanted with bilateral guide cannulae (26-gauge; Plastics One) located just above both OBs at the following coordinates with respect to bregma: anteroposterior,⫹5 mm; mediolateral, ⫾0.75 mm; dor-soventral,⫺0.7 mm with infusion cannulae extended 1 mm from the end of
the guide cannulae. Mice were allowed 10 d to recover from surgery in individual home cages with food and water ad libitum. Benzyloxycarbonyl-Val-Ala-Asp (ZVAD; 1g/l; in 1.5% DMSO; Calbiochem) or saline infu-sions (2l per OB) were performed after each visual training session.
Killing
One hour after the last behavioral trial (retention test of the olfactory association), 5 mice were randomly taken from each group, deeply anes-thetized (pentobarbital, 0.2 ml/30 g), and killed by intracardiac perfusion
Figure 1. a, Timing of the experiment. BrdU was administered 13 d before behavioral training. Mice were trained using an olfactory cue from D1 to D5, and then using a visual cue from D6 to D10. Retention of the olfactory task was tested on D14, and the animals were killed 1 h after the test. b, Behavioral suppression of an olfactory associative memory trace. Mice were first condi-tioned using an olfactory cue and then using a visual cue in the presence of the randomly presented odorant. Latency declined with sessions for olfactory (F(4,45)⫽ 7.86, p ⬍ 0.0001) and visual learning (F(4,45)⫽ 51.25, p ⬍ 0.0001), indicating that both were effective. Upon retention of the olfactory task, latency was higher than at D5 (D5 vs D14, p⬍0.0001).Acontrolgroupwassimilarly conditioned with the odorant (decrease in latency, F(4,45)⫽ 15.36, p ⬍ 0.0001) but underwent pseudo-conditioning with the visual cue in the absence of any odor (no decrease in latency, F(4,45)⫽ 0.21, p ⫽ 0.92). Latency during the retention test was not different from the last day of olfactory conditioning (D5 vs D14, p⬎ 0.05), indicating that the olfactory task was remembered. c, Olfactory pseudo-conditioned mice did not learn the associative task (no decrease in latency, F(4,45)⫽ 0.54, p ⫽ 0.7), while the second visual conditioning task in the presence of the random odorant was acquired (F(4,45)⫽86.84,p⬍0.0001).Latencyduring the retention test was similar to the end of pseudo-conditioning (D5 vs D14, p⬎ 0.05). d, Effect of visual conditioning in the absence of the random odor on retention of the olfactory task. Olfactory and visual conditioning were acquired (F(4,40)⫽1.62,p⬍ 0.0001; and F(4,40)⫽ 45.15, p ⬍ 0.0001, respectively). Latency during the retention test was similar to that on the last day of olfactory conditioning (D5 vs D14, p⬎ 0.05), indicating that the olfactory task was remembered.
of 50 ml of 4% paraformaldehyde in phosphate buffer, pH 7.4. Their brains were removed, postfixed, cryoprotected in sucrose (20%), frozen rapidly, and then stored at⫺20°C before sectioning with a cryostat (Jung).
Assessment of neurogenesis
5-Bromo-2
⬘-deoxyuridine administration
5-Bromo-2⬘-deoxyuridine (BrdU) (Sigma) (50 mg/kg in saline, three times at 2 h intervals) was injected 13 d before the behavioral training began.
BrdU immunocytochemistry
The protocol has already been described in detail (Mandairon et al., 2006a). Brain sections were first incubated in Target Retrieval Solution (Dako) then incubated in a mouse anti-BrdU primary antibody (1:100,
Millipore Bioscience Research Reagents). Sec-tions were then incubated in a horse biotinyl-ated anti-mouse secondary antibody (1:200, Vector Laboratories).
BrdU-positive cell quantification
All cell counts were conducted blind with re-gards to mouse status. Data were collected using a mapping software (Mercator Pro, Ex-plora Nova), coupled with a Zeiss microscope. In the granule cell layer of the right OB, BrdU-positive cells were counted on 16 sections (14 m thick, 140 m intervals, n ⫽ 5 per group). The number of positive cells was divided by the surface of the region of interest to yield the densities of labeled cells (labeled profiles/ m2). Using the same method, BrdU-positive cells were counted on 7–10 sections (14m thick) of the dentate gyrus distributed along the anteroposterior axis. Densities were ana-lyzed using ANOVA and unilateral Student’s t tests for comparison of single pairs.
Double-labeling analysis
To determine the phenotype of BrdU-positive cells in the granule cell layer of the OB, BrdU/ NeuN double labeling was performed using a rat anti-BrdU (1:100, Harlan Sera-Lab) and a mouse anti-NeuN (1:500, Millipore Bioscience Research Reagents). For functional involve-ment of newborn neurons, BrdU/Zif268 and BrdU/c-fos double labeling was performed us-ing a rabbit anti-Zif268 antibody (1:1000, Santa Cruz Biotechnology) or a rabbit anti-c-fos antibody (1:5000, Santa Cruz Biotechnol-ogy). The appropriate secondary antibodies, coupled to Alexa Fluor 633 to reveal BrdU and Alexa Fluor 488 (Invitrogen) to reveal other markers, were used.
BrdU-positive cells were examined for cola-beling with NeuN, Zif268, or c-fos (80 –100 cells per animal, n⫽ 3–5 animals per group). The double-labeled cells were observed and an-alyzed by pseudo-confocal scanning micros-copy using a Zeiss microscope equipped with the ApoTome. The percentage of double-labeled cells was calculated for each group and compared using ANOVA followed by unilat-eral t tests.
Results
Mice were conditioned to use an olfactory
cue to retrieve a food reward. This
associ-ation between the food and the odor was
then broken by associating the reward with a visual cue; although
the odor was still present in the environment, it could no longer
be used to locate the reward (Fig. 1a).
First, a group of mice (group 1, n
⫽ 10; Fig. 1b) underwent
olfactory conditioning. The animals were trained over a 5 d
pe-riod to use an olfactory cue (⫹limonene) to dig in the hole
con-taining the reward. The other hole contained no odor and no
reward. On the sixth day, to suppress the memory of the
⫹li-monene–reward association, the mice were similarly trained with
a new task in which they had to use a visual cue, a small red
cylinder placed next to the hole containing the reward. During
this visual conditioning, the
⫹limonene was randomly put in one
of the two holes, and was therefore present yet had no predictive
Figure 2. a, Experimental groups (same as in Fig. 1). b, Representative BrdU-positive cell (bi) and newborn (BrdU-positive) cell counts (bii) in the granule cell layer of the OB. Between-group differences were found (F(3,14)⫽12.22,p⬍0.0001).BrdU-positive cell density was higher in the groups remembering the associative olfactory task (groups 2 and 4) compared with the groups that forgot the task or had not learned it (groups 1 and 3). *p⬍ 0.05; **p ⬍ 0.01; ***p ⬍ 0.001. c, Neuronal differentiation of newborn cells was assessed by BrdU/NeuN double labeling (ci) and was similar in all four experimental groups (cii) (F(3.8)⫽ 0.74 p⫽0.56).d,ExampleofBrdU/Zif268doublelabeling(di)anddouble-labeledcellcounts(dii)showeddifferentialinvolvementof
value for the animal (Fig. 1b). Latency (the time to find the
re-ward) was measured as an index of performance. We found that
both behavioral tasks were rapidly acquired, as evidenced by the
decrease in latency (Fig. 1b). To assess whether the animals
re-membered the association
⫹limonene/reward, they were tested
on day 14 (D14), 9 d after the end of the olfactory conditioning in
four trials in which the odor cue once again signaled the reward.
The results of this retention test indicated that the animals did not
recall the olfactory task as shown by the latency, which was higher
than at the end of olfactory conditioning (D14 vs D5; Fig. 1b).
Using a second cohort of animals, we tested that this memory
erasure was due to the second learning and was not simply due to
the time between the olfactory conditioning and the retention
test. A control group (group 2, n
⫽ 10) was similarly conditioned
with the olfactory cue but then pseudo-conditioned with the
vi-sual cue (object randomly associated with the reinforced hole) in
the absence of any odor (Fig. 1b). In this group, the olfactory task
was retained as evidenced by the low latency observed during the
retention test, which was similar to that of the end of olfactory
conditioning (D14 vs D5; Fig. 1b). From these data, we concluded
that the second conditioning was efficient in breaking the odor–
reward association and producing behavioral extinction of the
memory trace. This paradigm was thus suitable for challenging
the hypothesis that newborn neurons are suppressed with
alter-ations to associative memory.
To assess neurogenesis, newborn neurons were counted in the
granule cell layer of the OB using BrdU cell labeling (Fig. 1a).
BrdU was injected 13 d before the beginning of the behavioral
testing to label a cohort of newborn cells present in the OB at the
beginning of the first task during their critical period for
experience-dependent survival (Mouret et al., 2008).
Impor-tantly, we found that the BrdU-positive cell density was
signifi-cantly decreased in those animals who had forgotten the olfactory
association compared with the control group who had
remem-bered it (Fig. 2bi,bii, compare groups 1, 2). Double labeling of
BrdU-positive cells with the neuronal marker NeuN (Fig. 2ci)
indicated that newborn cells were neurons at 90% in both groups
(Fig. 2cii). These results showed that breaking the odor–reward
association induced a reduction in newborn neurons, strongly
supporting the idea that they were indeed implicated in the
mem-ory trace and so died when the odor lost its associative value. In
addition, using double labeling of BrdU-positive cells with the
immediate early genes Zif268 and c-fos as indices of functional
integration in the network (Magavi et al., 2005; Moreno et al.,
2009; Sultan et al., 2010) (Fig. 2di,ei), we found that newborn
neurons were significantly less involved in processing the learned
odorant in the group that subsequently forgot it than in the group
that remembered the odor–reward association (Fig. 2dii,eii,
com-pare groups 1, 2). This finding suggests that the newborn neurons
that disappeared from the OB when the task was forgotten were
preferentially involved in processing the learned odor.
To further document this point, we compared neurogenesis in
these first two groups with a third group of animals (group 3, n
⫽
10) that was first pseudo-conditioned with the odor cue (odorant
randomly associated with the reward) and then underwent the
visual conditioning with the odor cue randomly present (Fig. 1c).
As expected, the animals could not learn any odor–reward
asso-ciation, whereas the visual cue–reward association was normally
learned (Fig. 1c). We found that the level of BrdU-positive cells
(coexpressing NeuN at 92.07
⫾ 1.5%) (Fig. 2cii) and the
func-tional implication of these newborn neurons in processing the
odor were lower in this group compared with the group that
learned and remembered the odor–reward association (group 2),
and similar to the group that learned and then forgot it (group 1)
(Fig. 2dii,eii). Two conclusions can be drawn from these data.
First, in line with previous studies, they indicate that learning
increases the survival of newborn bulbar neurons (Alonso et al.,
2006). Second, due to the BrdU injection protocol where
homol-ogous cohorts of cells were labeled in all three groups, we can
conclude that newborn neurons absent in group 1 were those that
failed to survive after by breaking the odor–reward association,
yet had obviously been initially selected to survive during
learn-ing of the olfactory task and retrieved in group 2 (Fig. 2bii). These
results thus demonstrate that breaking the odor–reward
associa-tion induced a premature disappearance of newborn neurons
initially selected by learning for long-term survival. These
neuro-genic effects were specific to the OB since we found no change in
the level of neurogenesis in the dentate gyrus of the hippocampus
(data not shown).
Finally, to determine which component of the behavioral
con-text suppressed the odor–reward memory trace (new visual cue–
reward association and/or presence of the odor randomly
associated with the reward), a new group of animals (group 4, n
⫽
10) first learned the odor–reward association and then the visual
Figure 3. a, Behavioral suppression of the odor–reward association using⫹carvone as the olfactory cue. Mice were first submitted to the⫹carvone–reward conditioning then to extinc-tion (visual cue in the presence of the random odorant) (Extincextinc-tion). Latency declined with sessions for olfactory (F(4,36)⫽ 8.83, p ⬍ 0.0001) and visual (F(4,36)⫽ 15.99, p ⬍ 0.0001) learning, indicating that both types of learning occurred. Upon retention test of the olfactory task, latency was higher than at D5 (D14 vs D5, p⬍ 0.05). A control group was similarly conditioned but in the absence of any odor during visual conditioning (Control). Both olfactory (decrease in latency, F(4,32)⫽ 7.43, p ⬍ 0.0001) and visual conditioning (F(4,32)⫽ 6.14, p ⬍ 0.001) were effective. Latency during the retention test was not different from the last day of olfactory conditioning (D14 vs D5, p⬎ 0.05), indicating that the olfactory task was remem-bered. b, BrdU-positive cell density was higher in the Control group, which remembered the associative olfactory task, compared with the Extinction group, which forgot the task. c, BrdU-positive cell density was higher in the ZVAD-treated group, which remembered the associative task, compared with the saline group, which forgot the task. *p⬍ 0.05.
cue–reward association in the absence of the odor (Fig. 1d).
Re-tention testing showed that these animals remembered the
olfac-tory task after the end of the second conditioning, the latency to
find the reward remaining as low as at the end of the first
condi-tioning (D14 vs D5; Fig. 1d). In line with behavior, the survival of
newborn cells in this group (coexpressing NeuN at 85.18
⫾
4.31%; Fig. 2cii) was higher than in the group that did not
re-member the task (group 1), and was similar to the group that did
(group 2) (Fig. 2bii). The percentage of newborn cells expressing
Zif268 or c-fos in response to the learned odorant was also similar
to that of group 2 (Fig. 2dii,eii). Refusing the reliable predictive
value of the odor thus allowed suppression of the associative
memory trace and learning-induced neurogenesis.
To confirm these findings, we repeated
the experiment of group 1 and group 4
using another odorant (⫹carvone) as the
olfactory cue. Two new groups of animals
(n
⫽ 10 each) were submitted to an
olfac-tory conditioning followed by the visual
conditioning with or without
⫹carvone
being randomly associated with the
re-ward. Similarly to what was obtained with
⫹limonene, the group undergoing the
ex-tinction procedure could not retrieve the
reward during the olfactory task as shown
by the increased latency in the retention
test (Fig. 3a) and showed fewer surviving
newborn cells (Fig. 3b).
To confirm that newborn cells were
supporting the memory of the task, we
tested whether memory was retained if the
death of newborn cells was inhibited
dur-ing extinction. To that purpose, we
in-fused in the OB the caspase pan-inhibitor
ZVAD (n
⫽ 10) or saline (n ⫽ 10) after
each session of extinction training in
ani-mals previously trained to the olfactory
conditioning (similar to group 1), and
looked at the retention of the olfactory
task on D14 (Fig. 4a). Saline-infused
ani-mals showed no retention of the task, as
shown by the rise in latency to find the
reward, confirming results obtained in
group 1 (Fig. 4b). In contrast, in
ZVAD-treated animals latency was not different
from the last day of olfactory training,
in-dicating that these animals remembered
the odor–reward association (Fig. 4b). In
parallel, we verified that ZVAD-treated
OBs retained more newborn cells than
saline-treated OBs (Fig. 4c). Thus,
protec-tion of the pool of newborn cells from
death leads to the persistence of the
mem-ory of the task.
Discussion
Together, the present data demonstrate
that newborn neurons participate in the
network that processes a learned odorant.
Indeed, they are removed from the
net-work when the memory trace is no longer
active, indicating that they support
long-term plasticity of the neural
representa-tion of an odor and the underlying
memory of its behavioral significance. Conversely, preventing the
death of newborn cells allowed the memory trace to persist
de-spite a behavioral procedure of extinction. This latter finding
suggests that forgetting the odor–reward association required the
disappearance of those newborn cells selected to survive during
learning of the association, strengthening the conclusion that
newborn cells support memory. Recent studies addressing the
role of newborn neurons in olfactory memory used
intervention-ist techniques to reduce neurogenesis yet failed to produce
con-vergent evidence for the implication of newborn neurons in
memory (Imayoshi et al., 2008; Breton-Provencher et al., 2009;
Lazarini et al., 2009; Sultan et al., 2010). In the present study,
Figure 4. Behavioral suppression of the olfactory associative memory trace was prevented by ZVAD infusions in the OB. a, Animals were implanted beforehand with bilateral cannulae in the OB, and ZVAD was infused after each session of extinction. b, Behavioral data. In the two groups, olfactory (decrease in latency: Saline, F(4,36)⫽ 8.53, p ⬍ 0.0001; ZVAD, F(4,36)⫽ 23.28, p ⬍ 0.0001) and visual (decrease in latency: Saline, F(4,36)⫽ 5;17, p ⬍ 0.005; ZVAD, F(4,36)⫽ 3.20, p ⬍ 0.05) conditioning were effective. Latency in the retention test increased compared with the last day of olfactory training in the Saline group (D14 vs D5,
using this new approach targeted at the behavioral alteration of
memory, we have been able to demonstrate that newborn
neu-rons selected by learning for long-term survival prematurely
dis-appeared when the olfactory memory trace is erased. Newborn
neurons in the present study were in their critical period for full
integration to the bulbar network at the time of learning
(Carle-ton et al., 2003). During this period, their survival rate is known
to be influenced by learning within a few days. In close temporal
correlation with the learning process, this regulation occurs
through apoptotic, caspase-dependent mechanisms (Sultan et
al., 2011). Our data further indicated that rapid and strong
mod-ulation of newborn cell death can also occur upon memory
era-sure. The cellular mechanisms underlying this postlearning
regulation of cell death are also likely to involve apoptotic
mech-anisms since the caspase pan-inhibitor treatment was efficient in
preventing death. These data strongly suggest that behavioral
ex-tinction triggered the death of newborn cells previously selected
to survive, allowing in turn memory erasure.
Rapid changes in newborn neuron survival could thus
under-lie rapid modulation in memory strength, which could be of
particular biological relevance to ensure that only current odor–
food associations are retained while no longer informative cues
lose their associative significance.
Together, the present findings highlight the adaptability of
newborn cell survival when modulation is required to adjust the
bulbar network for changes in the learned significance of an odor.
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